The present invention relates to a toner image fixing device in an electrophotographic image forming apparatus such as an electrophotographic printer.
In conventional electrophotographic printers, an electrostatic latent image is formed by an LED head on a surface of a photosensitive drum which has been charged uniformly in advance, and the electrostatic latent image is developed into a toner image by a developing roller. Then, the toner image on the photosensitive drum is transferred to paper by a transfer roller, and the toner image is fixed on the paper by a toner image fixing device.
FIG. 1 is a schematic diagram showing a conventional toner image fixing device. As shown in FIG. 1, a toner image fixing device 57 includes a heating roller 17a which contains a heater 58, a backup roller 17b, a surface of which is in contact with a surface of the heating roller 17a, and a temperature sensor 60 which is in contact with the surface of the heating roller 17a.
The heating roller 17a and the backup roller 17b are kept at a set temperature by the heater 58 before the fixing process. When the paper 16 passes through a fixing position formed between the heating roller 17a and the backup roller 17b, toner 19 constituting the toner image is heated and pressed. As a result, the toner image is fixed on the paper 16.
When the electric power supply switch (not shown in FIG. 1) of the electrophotographic printer is turned on and the current begins to flow from the power source (not shown in FIG. 1) to the heater 58, temperature of the heating roller 17a and the backup roller 17b is still equal to ambient temperature. In the conventional printers, two methods to be described below have been used to raise the temperatures of the heating roller 17a and the backup roller 17b to a set temperature.
FIG. 2A is a diagram showing temperature curves of the heating roller when a first and a second conventional methods of raising the temperature of the heating roller 17a are adopted, FIG. 2B is a diagram showing maximum and minimum temperature curves of the backup roller when the first method is adopted, and FIG. 2C is a diagram showing maximum and minimum temperature curves of the backup roller when the second method is adopted. Further, FIG. 3 is a diagram showing surface temperature distribution of the backup roller when either the first or second method is adopted.
In FIGS. 2A, 2B and 2C, horizontal axes represent time. In FIG. 2A, a vertical axis represents a surface temperature of the heating roller 17a, and in FIGS. 2B and 2C, vertical axes represent a surface temperature of the backup roller 17b. In FIG. 3, a horizontal axis represents a position of the backup roller 17b in the circumferential direction, and a vertical axis represents a surface temperature of the backup roller 17b.
In FIG. 2A, a curve L1 indicates the surface temperature of the heating roller 17a in the first method, and a curve L2 indicates the surface temperature of the heating roller 17a in the second method. In FIG. 2B, a curve L1.sub.MAX and a curve L1.sub.MIN indicate the highest temperature and the lowest temperature on the backup roller 17b, respectively, in the first method. In FIG. 2C, a curve L2.sub.MAX and a curve L2.sub.MIN indicate the highest temperature and the lowest temperature on the backup roller 17b, respectively, in the second method. The maximum temperature curves L1.sub.MAX and L2.sub.MAX in FIGS. 2B and 2C represent the temperatures on a contacting portion S.sub.1 (shown in FIG. 3) of the heating roller 17a and the backup roller 17b, while the minimum temperature curves L1.sub.MIN and L2.sub.MIN represent the temperatures at the location being farthest (180 degrees different in phase) away from the contacting portion S.sub.1 on the circumference of the backup roller 17b.
In the first method, when the electric power supply switch of the electrophotographic printer is turned on and the current begins to flow into the heater 58, the heating roller 17a and the backup roller 17b are still not rotated. After that, when the temperature sensor 60 detects a set temperature t.sub.s, the heating roller 17a and the backup roller 17b are rotated for a predetermined period of time, which is referred to as an equalizing time. This equalizing time is determined in such a way that the surface temperature distribution of the backup roller 17b is made uniform and the surface temperature of the backup roller 17b exceeds the set temperature.
In the second method, when the electric power supply switch of the electrophotographic printer is turned on and the current begins to flow into the heater 58, the heating roller 17a and the backup roller 17b are still not rotated. When the set time T.sub.1 elapses, the heating roller and the backup roller 17b are rotated for a predetermined equalizing time.
In the above-mentioned conventional methods, the rotation speeds of the heating roller 17a and the backup roller 17b during the equalizing time are set to be identical to those during the printing operation of the electrophotographic printer.
In the above-mentioned conventional methods, however, the heating roller 17a and the backup roller 17b are not rotated until the temperature sensor 60 detects the set temperature t.sub.s or the set time T.sub.1 elapses. For this reason, the temperature on the contacting portion S.sub.1 alone on the circumference of the backup roller 17b rises abruptly, as shown in FIG. 3.
In order to prevent this, the heating roller 17a and the backup roller 17b are rotated at ordinary rotation speeds for the equalizing time, as described above. Nevertheless, a difference between the temperature on the contacting portion S.sub.1 and the temperature on a portion other than the contacting portion S.sub.1 remains for a long time, as shown in FIG. 3. For this reason, if the equalizing time is short, a variation in the surface temperature of the backup roller 17b in the circumferential direction is large, thus resulting in the degradation of the image quality. Moreover, since a difference between the temperature on the contacting portion S.sub.1 and the temperature on the portion other than the contacting portion S.sub.1 is great, the life of the backup roller 17b is shortened.
In contrast therewith, if the equalizing time is long, the surface temperature of the backup roller 17b in the circumferential direction becomes rather uniform. However, it takes a longer time for the electrophotographic printer to be started up.
The method of initiating the rotation of the heating roller 17a and the backup roller 17b at a constant speed immediately after the electric power supply switch of the electrophotographic printer is turned on and the current flows into the heater 58 may also be conceived. In this method, however, heat diffusion from the heating roller 17a to the backup roller 17b increases, so that it takes a longer time to raise the surface temperature of the heating roller 17a to the set temperature. Accordingly, it also takes a longer time for the electrophotographic printer to finish a starting-up operation required before the fixing process.